U.S. patent application number 11/172527 was filed with the patent office on 2006-02-09 for intraluminal medical device having asymetrical members of unequal length.
Invention is credited to Robert Burgermeister, Randy Grishaber, Mathew Krever, Ramesh Marrey, David Overaker, Jin Park.
Application Number | 20060030930 11/172527 |
Document ID | / |
Family ID | 35478378 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060030930 |
Kind Code |
A1 |
Burgermeister; Robert ; et
al. |
February 9, 2006 |
Intraluminal medical device having asymetrical members of unequal
length
Abstract
This invention relates generally to expandable intraluminal
medical devices for use within a body passageway or duct, and more
particularly to an optimized stent having asymmetrical strut and
loop members, wherein at least one pair adjacent radial strut
members have unequal axial lengths.
Inventors: |
Burgermeister; Robert;
(Bridgewater, NJ) ; Grishaber; Randy; (Asbury,
NJ) ; Marrey; Ramesh; (Basking Ridge, NJ) ;
Park; Jin; (Parsippany, NJ) ; Krever; Mathew;
(Warren, NJ) ; Overaker; David; (Annandale,
NJ) |
Correspondence
Address: |
PHILIP S. JOHNSON;JOHNSON & JOHNSON
ONE JOHNSON & JOHNSON PLAZA
NEW BRUNSWICK
NJ
08933-7003
US
|
Family ID: |
35478378 |
Appl. No.: |
11/172527 |
Filed: |
June 30, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60584454 |
Jun 30, 2004 |
|
|
|
Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F 2002/91583
20130101; A61F 2002/91558 20130101; A61F 2002/91533 20130101; A61F
2250/0036 20130101; A61F 2/91 20130101; A61F 2002/91508 20130101;
A61F 2/915 20130101 |
Class at
Publication: |
623/001.15 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1. A stent comprising: one or more hoop components having a tubular
configuration with proximal and distal open ends defining a
longitudinal axis extending there between, each hoop component
being formed from a plurality of radial strut members, and one or
more radial arc members connecting adjacent radial struts, wherein
at least one pair of adjacent radial strut members has unequal
axial lengths from one another, and wherein at least one radial arc
member has non-uniform cross-sections to achieve near-uniform
strain distribution along the radial arc when the radial arc
undergoes deformation.
2. The stent of claim 1 wherein the cross-sections of the radial
arc members have substantially equivalent cross-sectional
areas.
3. The stent of claim 1 wherein the cross-sections of the radial
arc members have substantially non-equivalent cross-sectional
areas.
4. A stent comprising: one or more hoop components having a tubular
configuration with proximal and distal open ends defining a
longitudinal axis extending there between, each hoop component
being formed from a plurality of radial strut members, and one or
more radial arc members connecting adjacent radial struts, wherein
at least one pair of adjacent radial arc members has different
geometry from one another, and wherein at least one radial arc
member has non-uniform cross-sections to achieve near-uniform
strain distribution along the radial arc when the radial arc
undergoes deformation.
5. The stent of claim 4 wherein the different geometry of the
radial arc members comprise different arc cross-sections.
6. The stent of claim 4 wherein the different geometry of the
radial arc members comprise different arc radii.
7. The stent of claim 4 wherein the different geometry of the
radial arc members comprise different arc lengths.
8. A stent comprising one or more radial support members having at
least one radial component, wherein at least one pair of
circumferentially adjacent radial components has different geometry
from one another, and wherein the at least one radial component has
non-uniform cross-sections to achieve near-uniform strain
distribution along the radial component when the radial component
undergoes deformation.
9. The stent of claim 8 wherein the at least one radial component
is a radial arc member.
10. The stent of claim 9 wherein the cross-sections of the radial
arc member have substantially equivalent cross-sectional areas.
11. The stent of claim 9 wherein the cross-sections of the radial
arcs have non-equivalent cross-sectional areas.
12. The stent of claim 8 wherein the at least one radial component
is a radial strut member.
13. The stent of claim 12 wherein the cross-sections of the radial
strut have substantially equivalent cross-sectional areas.
14. The stent of claim 12 wherein the cross-sections of the radial
strut have non-equivalent cross-sectional areas.
15. A stent comprising one or more members each having a plurality
of components, wherein at least one pair of circumferentially
adjacent components has different geometry from one another, and
wherein the at least one component has non-uniform cross-sections
to achieve near-uniform strain distribution along the component
when the component undergoes deformation.
16. The stent of claim 15 wherein the component cross-sections have
equivalent cross-sectional areas.
17. The stent of claim 15 wherein the component cross-sections have
non-equivalent cross-sectional areas.
18. A stent comprising: A plurality of hoop components having a
tubular configuration with proximal and distal open ends defining a
longitudinal axis extending there between, each hoop component
being formed as a continuous series of substantially longitudinally
oriented radial strut members and a plurality of radial arc members
connecting adjacent radial struts, wherein at least one pair of
circumferentially adjacent radial strut members has unequal axial
lengths from one another; and one or more substantially
circumferentially oriented flex connectors connecting
longitudinally adjacent hoop components, each flex connector
comprising a flexible strut, with the flexible strut being
connected at each end by one flexible arc.
19. The stent of claim 18 wherein at least one radial strut member
is shaped to provide a greater cross-section.
20. The stent of claim 19 wherein the shape is a bulge.
21. The stent of claim 18 wherein the flexible strut and flexible
arcs comprising the flex connectors are arranged in a substantially
"S" configuration.
22. A stent comprising; a plurality of hoop components having a
tubular configuration with proximal and distal open ends defining a
longitudinal axis extending there between, wherein each hoop
component is formed from a plurality of circumferential hoop
sections, each hoop section being formed from a plurality of radial
strut members, and a plurality of radial arc members connecting
adjacent radial struts, wherein at least one pair of
circumferentially adjacent radial strut members has unequal axial
lengths from one another; and one or more substantially
circumferentially oriented flex connectors connecting
longitudinally adjacent hoop sections at one radial arc per hoop
section, wherein the circumferential amplitude of the flexible
connector is at least 1.5 times greater than the circumferential
amplitude of the connected radial arc.
23. The stent of claim 22 wherein each flex connector comprises a
flexible component.
24. The stent of claim 23 wherein the flexible component comprises
a flexible strut, with the flexible strut being connected at each
end by one flexible arc.
25. The stent of claim 22 having a plurality of flex connectors,
wherein each flex connector is shaped so as to nest together into
the circumferentially adjacent flex connector.
26. The stent of claim 22, wherein the unequal axial lengths of the
at least one pair of circumferentially adjacent radial strut
members allows the adjacent radial arc to nest into the
circumferentially adjacent radial strut.
27. The stent of claim 22, wherein the flex connector
circumferential amplitude is greater than the axial length of the
flex connector.
28. The stent of claim 24 wherein the flexible arc has non-uniform
cross-sections to achieve near-uniform stress distribution along
the flex component when the flex component undergoes
deformation.
29. A stent comprising: A plurality of hoop components having a
tubular configuration with proximal and distal open ends defining a
longitudinal axis extending there between, wherein each hoop
component is formed from a plurality of circumferential hoop
sections, and wherein corresponding points on longitudinally
adjacent hoop sections are circumferentially displaced from one
another.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority pursuant to 35 U.S.C.
.sctn. 119 (e) to provisional application 60/584,454 filed on Jun.
30, 2004.
FIELD OF THE INVENTION
[0002] This invention relates generally to expandable intraluminal
medical devices for use within a body passageway or duct, and more
particularly to an optimized stent having asymmetrical strut and
loop members, wherein at least one pair of circumferentially
adjacent radial strut members have unequal axial lengths.
BACKGROUND OF THE INVENTION
[0003] The use of intraluminal prosthetic devices has been
demonstrated to present an alternative to conventional vascular
surgery. Intraluminal prosthetic devices are commonly used in the
repair of aneurysms, as liners for vessels, or to provide
mechanical support and prevent the collapse of stenosed or occluded
vessels.
[0004] Intraluminal endovascular prosthetics involve the
percutaneous insertion of a generally tubular prosthetic device,
such as a stent, into a vessel or other tubular structure within
the vascular system. The stent is typically delivered to a specific
location inside the vascular system in a low profile (pre-deployed)
state by a catheter. Once delivered to the desired location, the
stent is deployed by expanding the stent into the vessel wall. The
expanded stent typically has a diameter that is several times
larger than the diameter of the stent in its compressed state. The
expansion of the stent may be performed by several methods known in
the art, such as by a mechanical expansion device (balloon catheter
expansion stent) or by self-expansion.
[0005] The ideal stent utilizes a minimum width and wall thickness
of the stent members to minimize thrombosis at the stent site after
implantation. The ideal stent also possess sufficient hoop strength
to resist elastic recoil of the vessel. To fulfill these
requirements, many current tubular stents use a multiplicity of
circumferential sets of strut members connected by either straight
longitudinal connecting connectors or undulating longitudinal
connecting connectors.
[0006] The circumferential sets of strut members are typically
formed from a series of diagonal sections connected to curved or
arc sections forming a closed-ring, zig-zag structure. This
structure opens up as the stent expands to form the element in the
stent that provides structural support for the vessel wall. A
single strut member can be thought of as a diagonal section
connected to a curved section within one of the circumferential
sets of strut members. In current stent designs, these sets of
strut members are formed from a single piece of metal having a
uniform wall thickness, generally uniform strut width, as well as
struts with uniform axial lengths. Similarly, the curved loop
members are formed having a generally uniform wall thickness and
generally uniform width.
[0007] Although the geometry of the stent members may be uniform,
the strain experienced by each member under load is not. The
"stress" applied to the stent across any cross section is defined
as the force per unit area. These dimensions are those of pressure,
and are equivalent to energy per unit volume. The stress applied to
the stent includes forces experienced by the stent during
deployment, and comprises the reactive force per unit area applied
against the stent by the vessel wall. The resulting "strain"
(deformation) that the stent experiences is defined as the
fractional extension perpendicular to the cross section under
consideration.
[0008] During deployment and in operation, each stent member
experiences varying load along its length. In particular, the
radial arc members are high in experienced loading compared to the
remainder of the structure. When the stent members are all of
uniform cross-sectional area, the resultant stress, and thus
strain, varies. Accordingly, when a stent has members with a
generally uniform cross-section, some stent members will be over
designed in regions of lesser induced strain, which invariably
results in a stiffer stent. At a minimum, each stent member must be
designed to resist failure by having the member size (width and
thickness) be sufficient to accommodate the maximum stress and/or
strain experienced. Although a stent having strut or arc members
with a uniform cross-sectional area will function, when the width
of the members are increased to add strength or radio-opacity, the
sets of strut members will experience increased stress and/or
strain upon expansion. High stress and/or strain can cause cracking
of the metal and potential fatigue failure of the stent under the
cyclic stress of a beating heart.
[0009] Cyclic fatigue failure is particularly important as the
heart beats, and hence the arteries "pulse", at typically 70 plus
times per minute--some 40 million times per year--necessitating
that these devices are designed to last in excess of 10.sup.8
loading cycles for a 10-year life. Presently, designs are both
physically tested and analytically evaluated to ensure acceptable
stress and strain levels are achievable based on physiologic
loading considerations. This is typically achieved using the
traditional stress/strain-life (S-N) approach, where design and
life prediction rely on a combination of numerical stress
predictions as well as experimentally-determined relationships
between the applied stress or strain and the total life of the
component. Fatigue loading for the purpose of this description
includes, but is not limited to, axial loading, bending,
torsional/twisting loading of the stent, individually and/or in
combination. One of skill in the art would understand that other
fatigue loading conditions can also be considered using the fatigue
methodology described as part of this invention.
[0010] Typically, finite-element analysis (FEA) methodologies have
been utilized to compute the stresses and/or strains and to analyze
fatigue safety of stents for vascular applications within the human
body. This traditional stress/strain-life approach to fatigue
analysis, however, only considers geometry changes that are uniform
in nature in order to achieve an acceptable stress and/or strain
state, and does not consider optimization of shape to achieve near
uniform stress and/or strain along the structural member. By
uniformity of stresses, a uniformity of "fatigue safety factor" is
implied. Here fatigue safety factor refers to a numerical function
calculated from the mean and alternating stresses measured during
the simulated fatigue cycle. In addition, the presence of flaws in
the structure or the effect of the propagation of such flaws on
stent life are usually not considered. Moreover, optimization of
the geometry considering flaws in the stent structure or the effect
of the propagation of such flaws has not been implemented.
[0011] What is needed is a stent design where the structural
members experience near uniform stress and/or strain along the
member, thereby maximizing fatigue safety factor and/or minimizing
peak strain, and analytical methods to define and optimize the
design, both with or without imperfections. One such resulting
design contemplates stent members with varying cross-sections and
strut members having different axial lengths. The design produces
near uniform stress and/or strain for a given loading condition
with or without the presence of defects or imperfections. The
design also allows for greater flexibility, conformability, and
offers a smaller crimping profile.
SUMMARY OF THE INVENTION
[0012] The present invention relates generally to expandable
intraluminal medical devices for use within a body passageway or
duct, and more particularly to an optimized stent having
asymmetrical strut and loop members, wherein at least one pair of
circumferentially adjacent radial strut members have unequal axial
lengths. In one embodiment of the present invention the stent has
one or more hoop components having a tubular configuration with
proximal and distal open ends defining a longitudinal axis
extending there between. Each hoop component is formed from a
plurality of radial strut members, and one or more radial arc
members connecting adjacent radial struts. At least one pair of
adjacent radial strut members have unequal axial lengths from one
another. In addition, at least one radial arc member has
non-uniform cross-sections to achieve near-uniform strain
distribution along the radial arc when the radial arc undergoes
deformation.
[0013] Another embodiment of the present invention includes a stent
comprising one or more hoop components having a tubular
configuration with proximal and distal open ends defining a
longitudinal axis extending there between. Each hoop component is
formed from a plurality of radial strut members, and one or more
radial arc members connecting adjacent radial struts. At least one
pair of the adjacent radial arc members have a different geometry
from one another. In addition, at least one radial arc member has
non-uniform cross-sections to achieve near-uniform strain
distribution along the radial arc when the radial arc undergoes
deformation.
[0014] In still another embodiment of the present invention, the
stent comprises one or more radial support members having at least
one radial component, wherein at least one pair of
circumferentially adjacent radial components has different geometry
from one another. In addition, at least one radial component has
non-uniform cross-sections to achieve near-uniform strain
distribution along the radial component when the radial component
undergoes deformation.
[0015] The present invention also includes a stent comprising one
or more members each having a plurality of components, wherein at
least one pair of circumferentially adjacent components has
different geometry from one another. In addition, at least one
component has non-uniform cross-sections to achieve near-uniform
strain distribution along the component when the component
undergoes deformation.
[0016] In still another embodiment of the invention the stent
comprises a plurality of hoop components having a tubular
configuration with proximal and distal open ends defining a
longitudinal axis extending there between. Each hoop component is
formed as a continuous series of substantially longitudinally
oriented radial strut members, and a plurality of radial arc
members connecting adjacent radial struts. At least one pair of
circumferentially adjacent radial strut members has unequal axial
lengths from one another. The stent further comprises one or more
substantially circumferentially oriented flex connectors connecting
longitudinally adjacent hoop components. Each flex connector
comprises a flexible strut, with the flexible strut being connected
at each end by one flexible arc.
[0017] Another embodiment of the invention comprises a stent having
a plurality of hoop components having a tubular configuration with
proximal and distal open ends defining a longitudinal axis
extending there between. Each hoop component is formed from a
plurality of circumferential hoop sections, where each hoop section
is formed from a plurality of radial strut members, and a plurality
of radial arc members connecting adjacent radial struts. At least
one pair of circumferentially adjacent radial strut members has
unequal axial lengths from one another. The stent further comprises
one or more substantially circumferentially oriented flex
connectors connecting longitudinally adjacent hoop sections at one
radial arc per hoop section. The circumferential amplitude of the
flexible connector is at least 1.5 times greater than the
circumferential amplitude of the connected radial arc.
[0018] In still another embodiment of the present invention, the
stent comprises a plurality of hoop components having a tubular
configuration with proximal and distal open ends defining a
longitudinal axis extending there between. Each hoop component is
formed from a plurality of circumferential hoop sections, and
corresponding points on longitudinally adjacent hoop sections are
circumferentially displaced from one another.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of an intraluminal stent in an
unexpanded or crimped, pre-deployed condition.
[0020] FIG. 2 is a perspective view of an intraluminal stent in the
fully expanded condition.
[0021] FIG. 3A is a front view illustrating a stent in its crimped,
pre-deployed state as it would appear if it were cut longitudinally
and then laid out into a flat in a 2-dimensional configuration
according to one embodiment of the present invention.
[0022] FIG. 3B is a magnified detail view of a proximal hoop
element according to one embodiment of the present invention.
[0023] FIG. 3C is a magnified detail view of a internal hoop
element according to one embodiment of the present invention.
[0024] FIG. 3D is a perspective view illustrating the nesting of
the flex connectors and hoop section components (radial arc and
radial strut) after crimping.
[0025] FIG. 3E is a magnified detail view of a flex connector
according to one embodiment of the present invention.
[0026] FIG. 3F illustrates a repeating sinusoidal wave pattern
having alternating high and low amplitude pairs according to one
embodiment of the present invention.
[0027] FIG. 3G illustrates a repeating sinusoidal wave pattern
according to one embodiment of the present invention.
[0028] FIG. 4A is a graphical representation of the
stress-intensity range (difference in stress intensity factors
across the fatigue loads) along the Y-axis versus the length of the
discontinuity along the X-axis.
[0029] FIG. 4B is a graphical representation of Fatigue Life of the
stent (along the Y axis) as a function of the discontinuity size
(along the X axis).
[0030] FIG. 5A is a magnified detail view of a stent section as
typically found in the prior art.
[0031] FIG. 5B is a magnified detail view of a stent section
according to one embodiment of the present invention.
[0032] FIG. 5C is a graphical representation of the strain
experienced by stent sections at various points along the stent
section.
DETAILED DESCRIPTION OF THE INVENTION
[0033] The present invention describes an intraluminal medical
device that will accommodate the device expansion into the wall of
a vessel lumen, while maintaining near uniform stress and/or strain
in the radial arcs when deployed. An intravascular stent will be
described for the purpose of example. However, as the term is used
herein, intraluminal medical device includes but is not limited to
any expandable intravascular prosthesis, expandable intraluminal
vascular graft, stent, or any other mechanical scaffolding device
used to maintain or expand a body passageway. Further, in this
regard, the term "body passageway" encompasses any duct within a
mammalian's body, or any body vessel including but not limited to
any vein, artery, duct, vessel, passageway, trachea, ureters,
esophagus, as well as any artificial vessel such as grafts.
[0034] The intraluminal device according to the present invention
may incorporate any radially expandable stent, including
self-expanding stents and mechanically expanded stents.
Mechanically expanded stents include, but are not limited to stents
that are radially expanded by and expansion member, such as by the
expansion of a balloon.
[0035] With reference to the drawing figures, like strut and arc
members are represented by like reference numerals throughout the
various different figures. By way of example, radial strut 108 in
FIG. 1 is equivalent to radial strut 308 in FIG. 3.
[0036] Referring to FIGS. 1 and 2, there is illustrated perspective
views of a stent 100 according to one embodiment of the present
invention. FIG. 1 illustrates the stent 100 in an unexpanded,
pre-deployed state, while FIG. 2 shows the stent 100 in the fully
expanded state.
[0037] The stent 100 comprises a tubular configuration of
structural elements having proximal and distal open ends 102, 104
and defining a longitudinal axis 103 extending there between. The
stent 100 has a first diameter D1 for insertion into a patient and
navigation through the vessels, and a second diameter D2 for
deployment into the target area of a vessel, with the second
diameter being greater than the first diameter.
[0038] The stent 100 structure comprises a plurality of adjacent
hoops 106(a)-(e) extending between the proximal and distal ends
102, 104. In the illustrated embodiment, the hoops 106(a)-(e)
encompass various radial support members and/or components. In
particular, the radial components that comprise the hoops
106(a)-(e) include a plurality of longitudinally arranged radial
strut members 108 (for example, 108b1, 108b2, 108b3 for hoop
106(b)) and a plurality of radial arc members 110 (for example,
110b1, 110b2 for hoop 106(b)) connecting adjacent radial struts
108. Circumferentially adjacent radial struts 108 are connected at
opposite ends in a substantially S or Z shaped pattern so as to
form a plurality of cells. The plurality of radial arc members 110
have a substantially semi-circular configuration and are
substantially symmetric about their centers.
[0039] The stent 100 structure further comprises a plurality of
flex connectors 114, which connect longitudinally adjacent hoops
106(a)-(e). Each flex connector 114 comprises one or more flexible
components. In the embodiment illustrated FIGS. 1 and 2, the
flexible components include one or more substantially
circumferentially oriented flexible strut members 116 and a
plurality of flexible arc members 118. Adjacent flexible struts 116
are connected at opposite ends in a substantially "S" shaped
pattern. The plurality of flexible arc members 118 have a
substantially semi-circular configuration and are substantially
symmetric about their centers.
[0040] Each flex connector 114 has two ends. One end of the flex
connector 114 is attached to one radial arc 110 (110a) on one hoop,
for examples hoop 106(c), and the other end of the flex connector
114 is attached to one radial arc 110 (110a) on a longitudinally
adjacent hoop, for example hoop 106(d). The flex connector 114
connects longitudinally adjacent hoops 106(a)-(e) together at "flex
connector to radial arc connection region" 117.
[0041] FIG. 3A illustrates a stent 300 according to one embodiment
of the present invention. The stent 300 is in its pre-deployed
state as it would appear if it were cut longitudinally and then
laid out flat in a 2-dimensional configuration. It should be
clearly understood that the stent 300 depicted in FIG. 3A is in
fact cylindrical in shape, similar to stent 100 shown in FIG. 1,
and is only shown in the flat configuration for the purpose of
illustration. This cylindrical shape would be obtained by rolling
the flat configuration of FIG. 3A and into a cylinder with the top
points "C" joined to the bottom points "D".
[0042] The stent 300 is typically fabricated by laser machining of
a cylindrical, Cobalt Chromium alloy tube. Other materials that can
be used to fabricate stent 300 include, other non-ferrous alloys,
such as Cobalt and Nickel based alloys, Nickel Titanium alloys,
stainless steel, other ferrous metal alloys, refractory metals,
refractory metal alloys, titanium and titanium based alloys. The
stent may also be fabricated from a ceramic or polymer
material.
[0043] Similar to FIG. 1, the stent 300 is comprised of a plurality
of cylindrical hoops 306 attached together by a plurality of flex
connectors 314. By way of example, a plurality of radial strut
members 308b (308b1, 308b2, 308b3) connected between radial arc
members 310b (310b1, 310b2) form a closed, cylindrical, hoop
section 306b (as shown within the dotted rectangle 312) in FIG.
3A.
[0044] A section of flex connectors 314 (as shown within the dotted
rectangle 326) bridge longitudinally adjacent hoop sections 306.
Each set of flex connectors 314 can be said to consist of a
multiplicity of substantially circumferentially oriented flexible
struts 316, with each flexible strut 316 being connected at each
end by one flexible arc 318 forming an "S" flexible connector
314.
[0045] In the illustrated embodiment, each hoop section 306 is
comprised of radial struts 308 and radial arcs 310 arranged in a
largely sinusoidal wave pattern having alternating amplitudes. It
should be noted that the amplitudes may repeat in some
predetermined pattern. For example, the internal hoop sections
(306(b), 306(c), etc.) have amplitudes that repeat in pairs. FIG.
3G illustrates a repeating sinusoidal wave pattern having
alternating high and low amplitude pairs according to one
embodiment of the present invention. For references purposes, an
imaginary reference line 375 is drawn perpendicular to the
longitudinal axis of the stent 300 midway between the extreme
positive and negative peaks of the sinusoidal wave pattern.
Progressing circumferentially along the internal hoop section, two
consecutive relatively high amplitudes 361 are followed by two
consecutive relatively low amplitudes 360.
[0046] Similarly, the end hoop sections (306(a, 306(c)) have
amplitudes that repeat in a 3 to 1 pattern. Specifically, FIG. 3G
illustrates a repeating sinusoidal wave pattern according to one
embodiment of the present invention. Progressing circumferentially
along the end hoop section, one relatively low amplitude 371 is
followed by three consecutive relatively high amplitudes 370.
[0047] Circumferentially adjacent flex connectors 314 are attached
to longitudinally adjacent hoops 306 every two complete sinusoidal
cycles. As a result, a given internal hoop section 306 has half the
number of flex connector attachment points 317 as radial arcs 310,
which results in a more flexible stent. FIG. 3E depicts a detail of
a typical flex connector 314 having a longitudinally oriented
flexible strut 316 connected at each end to a flexible arc 318. One
of skill in the art would understand that other repeating cycles
are contemplated by the present invention. For example, the
circumferentially adjacent flex connectors 314 may be attached to
longitudinally adjacent hoops 306 every three, four, etc. complete
sinusoidal cycles, or in some defined pattern.
[0048] Each "S" flex connector 314 is shaped so as to nest together
into the circumferentially adjacent S flex connector 314 as is
clearly illustrated in FIG. 3A. "Nesting" is defined as having the
top of a first flexible connector inserted beyond the bottom of a
second flexible connector situated just above that first flexible
connector. Similarly, the bottom of the first flexible connector is
inserted just below the top of a third flexible connector that is
situated just below that first flexible connector. Thus, a stent
with nested individual flexible connectors has each individual
flexible connector nested into both adjacent flexible connectors;
i.e., the flexible connector directly below and the flexible
connector directly above that individual flexible connector. This
nesting permits crimping of the stent 300 to smaller diameters
without having the "S" flex connectors 314 overlap. As described
earlier, the flex connector 314 configuration, where
circumferentially adjacent flex connectors 314 are attached to
longitudinally adjacent hoops 306 every two complete sinusoidal
cycles, thereby enhancing the ability of circumferentially adjacent
flex connectors 314 to nest during crimping.
[0049] In addition, the present design, utilizes variable amplitude
substantially sinusoidal patterns for nesting the hoop sections
during crimping. That is to say, the unconnected radial arcs 310
(310a1, 310b1, 310c1) will nest within the transition region
between the circumferentially adjacent medium length radial strut
308 and connected radial arc 310. FIG. 3D is a perspective view
illustrating the nesting of the flex connectors 314 and hoop
section 306 components (radial arc 310 and radial strut 308) after
crimping.
[0050] Stent 300 illustrated in FIG. 3A is comprised of 13 hoop
sections 306 connected by 12 sections of flex connectors 314. The
13 hoop sections 306 include 2 end hoop sections (proximal hoop
section 306a and distal hoop section 306c) and 11 internal hoop
sections 306b.
[0051] The internal hoop sections 306b are connected at opposite
ends by the sections of flex connectors 314 in a defined pattern to
form a plurality of closed cells 320. The end hoop sections (306a
and 306c) are connected at one end to the adjacent internal hoop
section 306(b) by a section of flex connectors 314, and similarly
form a plurality of closed cells. Adjacent hoop sections 306 may be
oriented out of phase, as illustrated in FIG. 3A. That is to say, a
corresponding point on longitudinally adjacent hoop sections are
circumferentially displaced from one another. This configuration
allows for increased amplitudes of flex connectors, which enables
greater stent flexibility during delivery, and greater
conformability post deployment. Alternatively, the adjacent hoop
sections 306 may be oriented in phase.
[0052] As described above, each hoop section in the illustrated
embodiment is comprised of radial struts 308 and radial arcs 310
arranged in a largely sinusoidal wave pattern having alternating
amplitudes. Each repeating wave pattern forms a hoop element 322.
The hoop element repeats at each flex connector 314 forming the
hoop 306.
[0053] In one embodiment of the invention, the substantially
circumferentially oriented flex connectors 314 connect
longitudinally adjacent hoop sections 306 at one radial arc 310 per
hoop section. The circumferential amplitude of the flexible
connectors 314 are at least 1.5 times greater, in the unexpanded
and un-crimped condition, than the circumferential amplitude of the
connected radial arc 310. This allows for increased flexibility
during delivery, and increased conformability in a deployed state.
In addition to the foregoing, the flex connector 314 may have a
circumferential amplitude greater than the axial length of the flex
connector 314. This allows for an increased number of hoops 306 and
flex connectors 314 over a given length. This enables the stent 300
to have greater scaffolding, increased flexibility, and a more
uniform curvature when bending.
[0054] By way of example, FIG. 3A shows each hoop section 306 being
comprised of 4 hoop elements 322. However, the number of repeating
hoop elements 322 is not meant to limit the scope of this
invention. One of skill in the art would understand that larger and
smaller numbers of hoop elements may be used, particularly when
designing stents of larger and smaller diameter.
[0055] FIGS. 3B and 3C are magnified detail views of proximal hoop
element 322a and internal hoop element 322b according to an
embodiment of the present invention. The proximal end hoop element
322a is attached to the flex connector 314 along its distal end. A
distal end hoop element 322c (not shown in detail) is a mirror
image of proximal end hoop element 322a and attached to the flex
connector 314 along its proximal end. FIG. 3C illustrates a typical
internal hoop element 322b attached to adjacent flex connectors 314
along its proximal and distal ends.
[0056] As earlier described, hoop element 322 comprises a plurality
of radial struts 308 and radial arcs 310 arranged in a largely
sinusoidal wave pattern having varying amplitudes. To achieve the
varying amplitude wave pattern, the hoop elements 322 are, in
general, comprised of radial struts 308 and radial arcs 310 of
varying dimensions within each hoop element 322. This design
configuration includes radial struts 308 having different lengths
and radial arcs 310 of different geometries. A stent having radial
struts of differing lengths is described in U.S. Pat. No. 6,540,775
to Fischell et al., dated Apr. 1, 2003 and is incorporated by
reference in its entirety herein. In addition, the proximal and
distal end hoop elements 322a and 322c are of a different
configuration than the internal hoop elements 322b. Accordingly,
the radial arcs 310 and radial strut 308 members that are part of
the internal hoop element 322b may be a different dimension that
the corresponding strut on the proximal or distal end hoop elements
322a and 322c respectively. The proximal and distal hoop elements
322a and 322c are mirror images of one another.
[0057] The intravascular stent must be circumferentially rigid and
possess sufficient hoop strength to resist vascular recoil, while
maintaining longitudinal flexibility. In typical sinusoidal and
near sinusoidal designs, the radial arcs experience areas of high
strain, and therefore stress, which are directly related to stent
fatigue. However, the stress and/or strain experienced along the
length of the radial arc is not uniform, and there are areas of
relatively low stress and/or strain. Providing a stent having
radial arcs with uniform cross-sectional results in areas of high
maximum stress and/or strain and other areas of relatively low
stress and/or strain. The consequence of this design is a stent
having lower expansion capacity.
[0058] The stent design according to the present invention has been
optimized around stress (fatigue safety factor) and/or strain,
which results in a stent that has near uniform strain, as well as
optimal fatigue performance, along the critical regions of the
stent. Optimal fatigue performance is achieved by maximizing the
near uniform fatigue safety factor along the stent. Various
critical regions may include the radial arcs 310 and/or radial
struts 308 and/or flexural arcs 318 and/or flexural struts 316. In
a preferred embodiment the critical region includes the radial arc
310. One method of predicting the stress and/or strain state in the
structure is finite element analysis (FEA), which utilizes finite
elements (discrete locations).
[0059] This design provides a stent having greater expansion
capacity and increased fatigue life. Where initial stress and/or
strain was high, material was added locally to increase the
cross-sectional area of the radial arc 310, and thereby distribute
the high local stress and/or strain to adjacent areas, lowering the
maximum stress and/or strain. In addition, changing the geometry of
the cross-section may also result in similar reductions to the
maximum stress and/or strain. These techniques, individually or in
combination (i.e. adding or removing cross-sectional area and or
changing cross-sectional geometry) are applied to the stent
component, for example, radial arc 310, until the resultant stress
and/or strain is nearly uniform. Another benefit of this design is
a stent having reduced mass.
[0060] The scope of this invention includes fracture-mechanics
based numerical analysis in order to quantitatively evaluate
pre-existing discontinuities, including flaws in the stent
structure, and thereby predict stent fatigue life. Further, this
methodology can be extended to optimize the stent design for
maximum fatigue life under the presence of discontinuities. This
fracture-mechanics based approach according to the present
invention quantitatively assesses the severity of discontinuities
in the stent structure including microstructural flaws, in terms of
the propensity of the discontinuity to propagate and lead to in
vivo failure of the stent when subjected to the cyclic loads within
the implanted vessel. Specifically, stress-intensity factors for
structural discontinuities of differing length, geometry, and/or
position of the discontinuity within and upon the stent structure
are characterized, and the difference in the stress intensities
associated with the cyclic loads are compared with the fatigue
crack-growth thresholds to determine the level of severity of the
discontinuity. Experimental data for fatigue crack-growth rates for
the stent material are then used to predict stent life based on the
loading cycles required to propagate the discontinuity to a
critical size.
[0061] FIG. 4A is a graphical representation of the
stress-intensity range (difference in stress intensity factors
across the fatigue loads) along the Y-axis versus the length of the
discontinuity along the X-axis. The solid line 480 represents the
threshold stress intensity range depicted as a function of
discontinuity length. This threshold stress range is characterized
for the given stent material. For a given stent design,
discontinuities of differing length, geometry, and/or position of
the discontinuity within and upon the stent structure are
numerically analyzed by introducing them into and/or onto the stent
structure, and the stress intensity ranges are computed for the
fatigue loads in question. By way of example, the dotted points
481-485 in FIG. 4A represent the calculated stress intensity ranges
for various discontinuity lengths. If these points 481-485 fall
below the threshold stress intensity curve 480 for a given
discontinuity length, the discontinuity is considered unlikely to
propagate during stent use, and in particular use during the long
term post deployment state. Conversely, if the points 481-481 fall
on or above curve 480, the discontinuity is more likely to
propagate during use.
[0062] Z A more conservative approach can be achieved by
numerically integrating the fatigue crack propagation relationship
for the given stent material between the limits of initial and
final discontinuity size. This approach disregards the existence of
threshold stress intensity range and is therefore considered more
conservative. The numerical integration results in predictions of
finite lifetimes for the stent as a function of discontinuity size.
FIG. 4B is a graphical representation of Fatigue Life of the stent
(along the Y axis) as a function of the discontinuity size (along
the X axis), and is characterized by curve 490.
[0063] Curve 490 is compared to the design life of the stent, curve
491, for additional assessment of stent safety. If the predicted
fatigue life 490 for a given discontinuity size is greater than the
design life 491, stents with these discontinuities are considered
safe. Conversely, if the predicted fatigue life 490 for a given
discontinuity size is less than or equal to the design life 491,
stents with these discontinuities are considered more susceptible
to failure during use.
[0064] FIGS. 5A through 5C may be used to compare the strain
experienced by the stent according to one embodiment of the present
invention to a typical prior art stent configuration. FIG. 5A shows
a magnified detail view of a radial arc 510a and adjacent radial
struts 508a (hereinafter stent section 530a) for a prior art stent.
As can be seen in the illustrated section 530a, the radial arc 510a
has a uniform width along its entire length.
[0065] FIG. 5B shows a similar magnified detail view of a radial
arc 510b and adjacent radial struts 508b (hereinafter stent section
430b) for a stent according to one embodiment of the present
invention. Unlike the prior art stent section 530a shown in FIG.
5A, the radial arc 510b has a non-uniform width to achieve near
uniform strain throughout the radial arc 510b.
[0066] In this description, strain optimization is being described
for the purpose of example. However, one of skill in the art would
understand that this method may also be applicable to optimize the
stress state as well.
[0067] For comparative purposes, the strain at five position points
(1 through 5) along each illustrated stent section 530 was measured
for a given expansion diameter. Position point 1 is located along
the radial strut 508. Position points 2 and 4 are located at each
root end of the radial arc 510, where the radial arc 410 connects
to the radial strut 508. Position point 3 is located along the
radial arc 510 at or near the apex or radial midpoint.
[0068] A graphical representation comparing the strain experienced
by the stent section 530a to the strain experienced by the stent
section 530b for a given expansion diameter is illustrated in FIG.
5C. The strain experienced by the prior art stent is identified in
the graph by curve C1 having non-uniform strain, with the strain
position points designated by a diamond shape. The total strain
experienced by the prior art sent section 530a is the area under
the curve C1.
[0069] The strain experienced by the stent according to one
embodiment of the present invention is identified in the graph by
the curve C2 having improved strain, with the strain position
points designated by a square. The total strain experienced by the
prior art sent section 530b is the area under the curve C2. Since
both stent sections 530a and 530b experience the same expansion,
the total strain is the same. That is to say, the area under the
curve C1 is the same as the area under the curve C2.
[0070] Turning to FIG. 5C, the strain experienced by the prior art
stent is relatively low at position points 1 and 2, reaching a
strain of approximately 8 at the root of radial arc 510a (position
point 2). The strain then increases dramatically to a maximum
strain of approximately 50% at position point 3, i.e. the apex of
radial arc 510a. The experienced strain is substantially symmetric
about the apex of the radial arc 510, dramatically decreasing to a
strain of approximately 8 at the root of the radial arc 510a
(position point 4), and nearly 0% at the radial strut 508a,
position point 5.
[0071] In comparison, the strain for the stent section 530b is
relatively low at position points 1, but increases more uniformly
between position points 2 and 3, reaching a strains of
approximately 18% at the root of the radial arc 510b (position
point 2) and 35% at the apex of radial arc 510b (position point 3).
Similar to curve C1, curve C2 is substantially symmetric about
position point 3.
[0072] As can be interpreted from FIGS. 5A through 5C, by modifying
the material cross-section (adding or subtracting material) from
the radial arc root (position points 2 and 4) the induced strain
was increased. This decreases the induced strain at the radial arc
apex (position point 3) since the total strain experienced by the
section remains unchanged. Further, by modifying the
cross-sectional area (adding or subtracting material) along the
apex of radial arc 510b (position point 3), the induced strain is
decreased. This automatically increases the induced strain at the
radial arc 510b roots (position points 2 and 4). These
modifications can be done individually as described, or in
combination, iteratively, to develop a stent section 530b having
improved near uniform strain along the radial arc 530b.
[0073] One advantage of having near uniform strain is that the peak
strain (shown at position point 3) is greatly reduced. As a result,
the stent may be expanded to a larger expansion diameter and still
be within safe operating levels of induced strain. For example, the
stent represented by curve C2 could be increased in diameter until
the peak strain at position point 3 is increased from 35% to
50%.
[0074] Returning again to FIGS. 3A through 3G, the stent 300
according to one embodiment of the present invention is laser cut
from a thin metallic tube having a substantially uniform wall
thickness. To vary the cross-section of the stent components,
particularly the radial arcs 310, the components have been tapered,
with larger widths in areas of high loading to achieve near uniform
stress and/or strain. It should be understood that the taper does
not have to be uniform, which is to say of a consistently changing
radius. Instead, the width of the radial arc 310 is dictated by the
resultant stress and/or strain experienced by the radial arc 310 at
various locations along its length.
[0075] FIGS. 3B and 3C show hoop elements 322 with tapered radial
arcs 310 and radial struts 308, according to one embodiment of the
present invention.
[0076] Turning to FIG. 3B, a proximal hoop element 322a is shown
according to one embodiment of the present invention. The hoop
element 322a is comprised of long and medium length radial struts,
308a1 and 308a2, respectively, and two different radial arcs 310a1
and 310a2. The differences in the two radial arcs may include,
different geometries, such as different arc cross-sections;
different arc radii; and different arc lengths. However, one of
skill in the art would understand that other geometric differences
are also contemplated by the present invention, and the identified
differences should not be meant to limit the scope of the
invention.
[0077] The use of the terms "long", "medium", "short" or
"different" are meant to describe relative differences between the
various components and not to connote specific or equivalent
dimensions.
[0078] FIG. 3C shows an internal hoop element 322b according to one
embodiment of the present invention. The hoop element 322b is
comprised of long, medium and short length radial struts, 308b1,
308b2, and 308b3 respectively, and two different radial arcs 310b1
and 310b2. The differences in the two radial arcs may include,
different geometries, such as different arc cross-sections;
different arc radii; and different arc lengths. However, one of
skill in the art would understand that other geometric differences
are also contemplated by the present invention, and the identified
differences should not be meant to limit the scope of the
invention.
[0079] Radial arc 310b1 connects medium radial strut 308b2 to small
radial strut 308b3, and is not connected to flex connector 314.
Similarly, radial arc 310b2 connects medium radial strut 308b2 to
long radial strut 308b1, and is connected to flex connector
314.
[0080] The stent design according to the present invention may also
be optimized around minimizing maximum stress and/or strain to
obtain a stent that has near uniform stress and/or strain at each
point along the flex connectors 314. This design will provide a
more flexible stent, having flex connector sections of smaller
cross-section where the initial measured load and stress and/or
strain were low. The aforementioned criteria (i.e. adding or
removing cross-section) is applied to the flex connector 314 until
the resultant stress and/or strain is nearly uniform.
[0081] The radial struts 308 experience relatively low stress
and/or strain compared to the flex connectors 314 and radial arcs
310, so tapering of the struts 308 is typically not necessary to
minimize maximum stress and/or strain for fatigue resistance.
However, increasing the cross-section of the radial struts 308 as
illustrated in FIGS. 3A through 3D makes the struts 308, and thus
the stent 300, more radio-opaque. This enhances the visibility of
the stent during fluoroscopic procedures. Increasing the
cross-section of the struts 308 may also include shaping or adding
a shape to the strut to increase the strut size. In one embodiment
a bulge shape 309 is added to the stent strut 308. However, one of
skill in the art would understand that the type of geometric shape
added to the strut 308 is not meant to limit the scope of the
invention.
[0082] Therapeutic or pharmaceutic agents may be applied to the
device, such as in the form of a drug or drug eluting layer, or
surface treatment after the device has been formed. In a preferred
embodiment, the therapeutic and pharmaceutic agents may include any
one or more of the following: antiproliferative/antimitotic agents
including natural products such as vinca alkaloids (i.e.
vinblastine, vincristine, and vinorelbine), paclitaxel,
epidipodophyllotoxins (i.e. etoposide, teniposide), antibiotics
(dactinomycin (actinomycin D) daunorubicin, doxorubicin and
idarubicin), anthracyclines, mitoxantrone, bleomycins, plicamycin
(mithramycin) and mitomycin, enzymes (L-asparaginase which
systemically metabolizes L-asparagine and deprives cells which do
not have the capacity to synthesize their own asparagine);
antiplatelet agents such as G(GP) II.sub.b/III.sub.a inhibitors and
vitronectin receptor antagonists; antiproliferative/antimitotic
alkylating agents such as nitrogen mustards (mechlorethamine,
cyclophosphamide and analogs, melphalan, chlorambucil),
ethylenimines and methylmelamines (hexamethylmelamine and
thiotepa), alkyl sulfonates-busulfan, nirtosoureas (carmustine
(BCNU) and analogs, streptozocin), trazenes-dacarbazinine (DTIC);
antiproliferative/antimitotic antimetabolites such as folic acid
analogs (methotrexate), pyrimidine analogs (fluorouracil,
floxuridine, and cytarabine), purine analogs and related inhibitors
(mercaptopurine, thioguanine, pentostatin and
2-chlorodeoxyadenosine {cladribine}); platinum coordination
complexes (cisplatin, carboplatin), procarbazine, hydroxyurea,
mitotane, aminoglutethimide; hormones (i.e. estrogen);
anticoagulants (heparin, synthetic heparin salts and other
inhibitors of thrombin); fibrinolytic agents (such as tissue
plasminogen activator, streptokinase and urokinase), aspirin,
dipyridamole, ticlopidine, clopidogrel, abciximab; antimigratory;
antisecretory (breveldin); anti-inflammatory: such as
adrenocortical steroids (cortisol, cortisone, fludrocortisone,
prednisone, prednisolone, 6.alpha.-methylprednisolone,
triamcinolone, betamethasone, and dexamethasone), non-steroidal
agents (salicylic acid derivatives i.e. aspirin; para-aminophenol
derivatives i.e. acetominophen; indole and indene acetic acids
(indomethacin, sulindac, and etodalac), heteroaryl acetic acids
(tolmetin, diclofenac, and ketorolac), arylpropionic acids
(ibuprofen and derivatives), anthranilic acids (mefenamic acid, and
meclofenamic acid), enolic acids (piroxicam, tenoxicam,
phenylbutazone, and oxyphenthatrazone), nabumetone, gold compounds
(auranofin, aurothioglucose, gold sodium thiomalate);
immunosuppressives: (cyclosporine, tacrolimus (FK-506), sirolimus
(rapamycin), azathioprine, mycophenolate mofetil); angiogenic
agents: vascular endothelial growth factor (VEGF), fibroblast
growth factor (FGF); angiotensin receptor blockers; nitric oxide
donors; anti-sense oligionucleotides and combinations thereof; cell
cycle inhibitors, mTOR inhibitors, and growth factor receptor
signal transduction kinase inhibitors; retenoids; cyclin/CDK
inhibitors; HMG co-enzyme reductase inhibitors (statins); and
protease inhibitors.
[0083] While a number of variations of the invention have been
shown and described in detail, other modifications and methods of
use contemplated within the scope of this invention will be readily
apparent to those of skill in the art based upon this disclosure.
It is contemplated that various combinations or subcombinations of
the specific embodiments may be made and still fall within the
scope of the invention. For example, the embodiments variously
shown to be cardiac stents may be modified to treat other vessels
or lumens in the body, in particular other regions of the body
where vessels or lumen need to be supported. This may include, for
example, the coronary, vascular, non-vascular and peripheral
vessels and ducts. Accordingly, it should be understood that
various applications, modifications and substitutions may be made
of equivalents without departing from the spirit of the invention
or the scope of the following claims.
[0084] The following claims are provided to illustrate examples of
some beneficial aspects of the subject matter disclosed herein
which are within the scope of the present invention.
* * * * *